The present invention concerns a novel sequence of catalytic processes, purification processes and separation processes for the production of high purity aromatic compounds such as benzene, ortho-xylene and para-xylene.
It also concerns a novel process for the treatment of hydrocarbon cuts which are rich in aromatic compounds, more particularly a process for the elimination of diolefins and the reduction and possible elimination of olefins from mixtures of aromatic compound-rich hydrocarbons using a hydrogenation reaction by means of particular catalysts.
More particularly, it concerns the production of very pure para-xylene, in particular for the synthesis of terephthalic acid which is mainly used in the textile industry.
Mixtures of aromatic compound-rich hydrocarbons are used in petrochemical plants which produce benzene, toluene, ethylbenzene, meta-xylene, ortho-xylene, para-xylene, ethylbenzene and styrene after various separation and purification treatments.
The sources of aromatic compound-rich cuts containing different concentrations of diolefins and olefins are generally: distillation processes for crude hydrocarbons such as crude oil, condensed natural gas, coal; thermal processes such as steam cracking of naphtha; processes dedicated to the production of aromatics from light aliphatic cuts (C.sub.3 -C.sub.5, more particularly C.sub.3 and C.sub.3 /C.sub.4), C.sub.6 and C.sub.6 /C.sub.7 aliphatics, and heavy naphtha cuts (&gt;C.sub.6 for various catalytic reforming processes); and processes for transforming aromatic products, such as processes for trans-alkylation and isomerisation of ortho- and meta-xylenes to para-xylene.
Catalytic reforming is the major process for producing mixtures which are rich in aromatic compounds. At first, catalytic reforming was carried out in two types of facilities, depending on whether it was to be used for refining or petrochemistry. Later on, this distinction, linked to the severity of the operating conditions, has become blurred.
Nowadays, in order to satisfy increased energy constraints, the industry is again seeking more specific processes. They thus use, for refining, catalytic reforming units which operate at high severity but which have greater operational stability and improved spirit yields, and for petrochemistry aromatic production (benzene, toluene and xylenes) is optimised by using a reactor which operates at low pressure.
The use of reformers operating at high severity is accompanied by an increase in the concentration of olefins and diolefins in the reformates. The bromine number of a stabilised reformate can reach a maximum value of over 7000 mg of bromine per 100 g of product for units operating at very low pressure. The presence of these olefins and diolefins is particularly prejudicial to aromatic separation processes. As an example, olefins and diolefins tend to polymerise in the solvents used for extraction. Purification treatments using natural silico aluminates, usually activated (for example attapulgite, bentonites and montmorillonites activated by treatment in the presence of acids) are currently used. Those purification materials are generally termed activated clays.
Purification treatments using activated clay have a number of problems, among them: a short lifetime (generally 4 to 6 months, sometimes as low as one month for feeds containing high concentrations of olefins, &gt;0.6% by weight), low catalytic activity (hourly space velocities in the range 0.5 to 3 volumes of feed/volume of catalyst/hour), and great difficulty in purifying feeds containing more than 1.5% by weight of olefins and diolefins, which limits the operating severity of the reformer. Purification of the feed is accompanied by the production of high molecular weight compounds due to alkylation of the aromatics. These products must then be separated out. They cannot be regenerated economically and are thus eliminated in the waste with their toxic residual aromatic products.
Conventional schemes for the production of aromatic compounds containing 6 to 8 carbon atoms per molecule are complex. They comprise a succession of separation, purification and catalytic process steps. An aromatic plant for the production of para-xylene and ortho-xylene can be described as comprising at least the sequence of steps described above.
The source of aromatics, i.e., the stabilised reformate, is introduced into a separating column to produce a C.sub.7.sup.- cut overhead which contains benzene and toluene, the column being currently termed a "deheptanizer". An aromatic C.sub.8.sup.+ cut is extracted from the bottom of the deheptanizer and purified by passage over at least one bed of activated clay. Purification is intended to eliminate the major portion of the olefins and diolefins from that C.sub.8.sup.+ cut. Since the lifetime of those activated clay beds is relatively short, two or more beds are positioned in parallel to allow a change from one to the other to enable discharge of the used clay without stopping the production of purified C.sub.8.sup.+.
Following the clay treatment, the aromatic C.sub.8.sup.+ cut is introduced into a separating column from which an aromatic C.sub.8 cut is extracted overhead (xylene+ethylbenzene) and a heavy fraction containing ortho-xylene and compounds with a higher molecular weight than ortho-xylene is extracted from the bottom. Normally, a second separation step is carried out on the latter cut and ortho-xylene is recovered overhead from the second column while an aromatic C.sub.9.sup.+ cut is recovered from the bottom, from which the C.sub.9 aromatics may be separated to produce, by transalkylation with benzene, an additional quantity of para-xylene and ortho-xylene.
The aromatic C.sub.8 cut obtained overhead from the first column, which contains the three isomers of xylene (ortho-xylene, meta-xylene and para-xylene) plus ethylbenzene, is sent to a xylene separation unit which uses, for example, molecular sieves or a crystallization process, from which para-xylene and an aromatic C.sub.8 cut are obtained. The latter is introduced into a catalytic isomerisation unit, possibly after addition of hydrogen, in which the xylenes and possibly ethylbenzene are isomerised to produce a C.sub.8 aromatic mixture at close to thermodynamic equilibrium, which thus contains para-xylene. One isomerisation scheme is described in United States patent U.S. Pat. No. 4,224,141.
Following the isomerisation reactor, a column for separating the light C.sub.5.sup.- fractions produced in the isomerisation reactor is generally used, followed by a further separating column from which a C.sub.8.sup.+ cut, containing C.sub.8 aromatics close to thermodynamic equilibrium and heavy aromatic compounds from the transalkylation reactions of the isomerisation process, is recovered from the bottom. The aromatic C.sub.8.sup.+ cut may contain olefins and diolefins. This cut is thus normally treated on at least one activated clay bed before recycling it to the deheptanizer which treats the reformate. A scheme of this type is described in French patent application FR 94/15896 which also comprises a para-xylene enrichment process by selective adsorption or low temperature crystallization, a step of purification by crystallization with at least one high temperature stage, and an isomerisation process. This requires the use of at least two reactors and the waste clay becomes more and more of a problem for reasons of environmental pollution.
United States patent U.S. Pat. No. 4,118,429 describes a process for the hydrogenation of olefins in effluents from units for the isomerisation of aromatic compounds using a metal catalyst selected from the following metals: ruthenium, rhodium, palladium, osmium, iridium, platinum, or a mixture of those metals. The stated aim of that invention is to improve recovery of para-xylene on treatment with the adsorbent. The example indicates that an olefin-free effluent is obtained after treatment at 177.degree. C. at a pressure of 9.7 bar, a space velocity of 3 h.sup.-1 and using a catalyst composed of 0.375% by weight of platinum on alumina. The aromatic compound loss is not explained in the description of that invention; however, the loss is significant. In fact, the hydrogenation of xylene-rich cuts by metals from group VIII leads to a significant loss of xylenes in the form of dimethylcyclohexanes, as will be indicated below.
Further, the effluent still contains monoolefins which adversely affect downstream treatment, in particular molecular sieve adsorption.